1. Field of the Invention
This invention pertains to chemical vapor deposition. It is particularly directed to methods and apparatus for chemical vapor deposition of a metal oxide film on a substrate.
2. Background
For convenience, terms derived from or related to the word "stoichiometry" are used in this disclosure variously as nouns, adjectives or other parts of speech. While this usage may not in every instance be grammatically precise, it is in harmony with common practice in the relevant art. All such terms should be understood in context to refer to compositions or materials, such as precursors, films or crystals, in which the constituent elements or moieties are present in an essentially stoichiometric ratio. For example, a thin film of essentially perfect crystalline composition may be referred to as having the correct "stoichiometry" or as being "stoichiometric."
Vapor deposition involves the evaporation or sublimation of a precursor material, and the subsequent condensation of selected constituents of the precursor onto a substrate. The formation of metallic or metal oxide films through physical vapor deposition techniques typically requires very high temperatures, on the order of 1200.degree.-2000.degree. C., to evaporate the coating material. Low pressures, generally less than 1 mm Hg, are also required. Chemical vapor deposition techniques may be conducted at lower temperatures and higher pressures, but tend to produce non-stoichiometric films. Chemical vapor deposition involves the destructive vaporization of a metalorganic precursor material, typically by pyrolysis.
Precursors which include multiple metal constituent materials tend to decompose when pyrolyzed, thereby producing a film containing non-stoichiometric ratios of the metals. For films to be useful for optical applications, they should be characterized by an essentially perfect crystal lattice structure. Flawed double metal oxide films form imperfect crystals which render them substantially useless for optical transmission in electro-optical devices.
Impurities, voids or non-stoichiometric structure in a crystal will create color centers and other optical inhomogeneities that alter the light or scatter it, destroying the effectiveness of the transmitted light beam. Any such imperfection will typically degrade the coherency or intensity of a transmitted beam.
According to the conventional sol gel process, a precursor, such as a metalorganic compound, is coated onto a substrate, for example, by dipping. The coating must then be heated to drive off the undesired organic constituents, leaving the desired film, typically a double metal oxide. The sol gel process leaves many and large voids that must be reduced by sintering. Sintering causes substantial reductions in film thickness, thereby producing a film of uneven thickness in which voids typically remain. Impurities and non-stoichiometric species, while fewer than typically result from pyrolytic deposition techniques, are nevertheless characteristically present.
Metal oxide chemical vapor deposition (MOCVD) of multicomponent oxide films has involved the use of separate metalorganic compounds containing the respective film components as precursors. The precursor compounds are ordinarily independently transformed to the vapor phase (by evaporation or other means) in a controlled manner to achieve the desired molar ratio between vapor phase species. Whether a given vapor phase composition results in the desired film composition may be determined as a practical matter only after a compositional analysis of the final film is performed. While this approach allows for convenient variation of compositions away from stoichiometric ratios, it inevitably requires a trial-and-error approach to achieve a precisely stoichiometric film composition.
U.S. Pat. No. 5,213,844 discloses volatile alkoxide precursors for chemical vapor deposition on substrates to produce films having superconductivity properties. U.S. Pat. No. 5,185,317 discloses the use of vaporized metal precursors in chemical vapor deposition procedures to achieve molecular level uniformity of coatings. U.S. Pat. Nos. 5,173,455; 5,139,976 and 4,835,298 disclose double alkoxides and their hydrolysis in the liquid state. U.S. Pat. No. 5,160,618 discloses chemical vapor deposition procedures for the growth of crystals of multi-component metal oxides on substrates. U.S. Pat. No. 5,112,676 discloses the use of precursor material containing metals in the same proportions as desired for a microwave oven metal oxide coating. U.S. Pat. No. 5,076,901 discloses metalorganic chemical vapor deposition of volatile metal precursors. U.S. Pat. No. 4,898,842 discloses the use of metalorganic precursors for forming metal oxide layers in electronic components. This patent also discusses known procedures for hydrolyzing double alkoxide precursors to homogeneous perovskites and spinels. U.S. Pat. No. 4,543,341 discloses the hydrolysis of double alkoxides in the formation of metal oxides. The disclosures of each of these patents are incorporated as a part of this disclosure for their teachings concerning the conventional operations incidentally involved in connection with the practice of this invention.
There has not heretofore been available any procedure for the practical application of vapor deposition techniques to metallo-organic precursors capable of crystal growth at low temperatures. It has not been known how to apply vapor deposition technology, notably vapor phase hydrolysis, to double alkoxide precursors to produce a homogeneous phase from which may be grown large crystals containing the same stoichiometric ratio of metals as exists in the precursor.
There remains a need for an improved vapor deposition technique capable of producing nearly stoichiometric metal oxide films suitable for use in optical applications. Such a technique would be expected to be useful in the production of other films useful for other applications.